The classical holographic optical element is a lens formed by recording the interference pattern from a plane wave and a converging spherical wave in a suitable medium like photographic film. When the hologram is read out using a plane wave, the light is focused at the same point as the original spherical wave, provided that the wavelength of the reconstructing light is the same as that of the light used to record the hologram and provided that the other relevant properties (mean refractive index of the film, for example) are unchanged. Unfortunately, most holographic lenses are meant to be used at free-space wavelengths that are somewhat longer (lower frequencies) than the free-space wavelengths (higher frequencies) at which good and available optical recording materials are most sensitive, so holograms must be recorded by light of different free-space wavelength (higher frequency) from that at which they are to be used (lower frequency). Since the angles at which light rays are diffracted from the local grating structure comprising the hologram are nonlinear functions of the fringe spacings and the optical wavelengths of the reconstructing light, the wavelength shift between recording and reconstructing a hologram introduces aberrations. This is further compounded by the fact that if high diffraction efficiencies are also required, thick (Bragg) phase holograms must be used; so, geometric constraints become more severe, since these holograms require that the light enter at prescribed angles.
Because holographic optical elements have important industrial and military applications such as in missile guidance, robotics, and automation, considerable effort has been directed to overcoming the aberrations attendant with wavelength shifts. A recent publication by K. Winick ("Designing Efficient Aberration-Free Holographic Lenses in the Presence of a Construction-Reconstruction Wavelength Shift," J. Opt. Soc. Am. 72, 143-148, 1982) details the design of diffraction-limited lenses that also operate correctly in the Bragg regime, to first order. Earlier publications by J. M. Moran ("Compensation of Aberrations Due to a Wavelength Shift in Holography," Applied Optics 10, 1909-1913, 1971) and by J. N. Latta ("Computer-Based Analysis of Hologram Imagery and Aberrations II: Aberrations Induced by a Wavelength Shift," Applied Optics 10, 609-618, 1971) laid the foundations for computer analysis to minimize aberrations in such elements. The results of the analyses are used to design geometries that will permit using competing aberrations to cancel one another.
For image reproducing systems, W. Friedl (U.S. Pat. No. 3,598,466 Aug. 10, 1971) uses a system of lenses to correct aberrations in a hologram produced with visible radiation and reconstructed with infrared radiation while L. Lin (U.S. Pat. No. 3,639,031, Feb. 1, 1972) records a first hologram at a first free space wavelength and then produces a second hologram from the first hologram using light of a second free space wavelength. The second hologram is read using light of the first free-space wavelength to produce an image with minimum aberration.
Although holographically-formed gratings have been used to achieve optical coupling between fiber-optic waveguides and planaroptic waveguides (J. Hammer, U.S. Pat. Nos. 3,912,363 Oct. 14, 1975 and 4,152,045, May 1, 1979 and S. Wright, U.S. Pat. No. 4,184,738, Jan. 22, 1980) and for wavelength multiplexing/demultiplexing (W. Tomlinson, et. al., U.S. Pat. No. 3,814,498, June 4, 1974), this prior art does not mention the reduction or elimination of chromatic aberration in holograms.